Organisms are engaged in signal transduction mutually between cells or tissues, thus maintaining well-balanced regulation of development, differentiation, proliferation, maintenance of homeostasis, etc. In many cases, proteinous factors mediate them. For example, many secretory factors (humoral factors) that participate in the immune system or the erythropoietic system were found and these factors are called cytokines. Lymphokines, monokines, interferons, colony-stimulating factors, tumor necrosis factors, etc. are included in cytokines. These cytokines have been actively studied on their involvement in diseases or applications to pharmaceuticals.
Humoral factors produced in endocrine tissues such as peptide hormones, growth factors, etc. also play some crucial roles for sustained homeostasis or growth and their application to medicaments have been extensively studied.
In view of their structural characteristics, insulin, insulin-like growth factor I (IGF-I), insulin-like growth factor II (IGF-II), relaxins H1 and H2 are considered to be humoral factors that constitute family members, and are responsible for a wide range of physiological roles in the body, including metabolic regulation of carbohydrates, growth promotion of tissues, control of reproductive functions, etc. [BioScience Term Library, Cytokine/Growth Factor, Yodosha, pages 108–109, 1995; Molecular Biology of Hormone 6, Development and Growth Factor/Hormone, Gakkai Shuppan Center, pages 1–23, 1996]. Moreover, new molecular species belonging to the same family are in the process of discovery [Molecular Endocrinology, 13, 2163–2174, 1999]. The structural characteristic of these precursor proteins lies in possessing the signal sequence-B domain-C domain-A domain structure. B and A domains, which are mature molecules, are connected via disulfide bonds. Furthermore, one disulfide bond located within the A domain is important for maintenance of the steric structure and expression of activities. Insulin, insulin-like growth factors and relaxins have a variety of physiological activities and are important factors for signal transduction in vivo. Insulin is secreted from β cells of the pancreas and important for regulation of energy metabolism, including promotion of sugar uptake in liver, muscle and adipose tissues, promotion of fatty acid synthesis, promotion of glycogen synthesis, etc. In addition, insulin has a cell proliferation activity. IGF-I is synthesized mainly in liver and promotes the proliferation or differentiation of many cells including bone-derived cells. IGF-II has a cell proliferation promoting activity and an insulin-like activity, like IGF-I. These factors bind to specific receptors to cause autophosphorylation of the receptors, which could be a trigger of a chain of subsequent reactions. Turning to relaxin, it was recently found to have a wide variety of physiological activities, including activities primarily associated with reproductive functions such as relaxation of birth canal, softening of connective tissues due to reconstruction of collagen, promotion of proliferation, differentiation of mammary gland, etc. These activities include, for example, promotion in dilation of blood vessels in uterus, mammary gland, lung, heart, etc., effects on heart strokes, inhibition of histamine release from mast cells, inhibition of platelet aggregation, secretory regulation of pituitary hormone, regulation of body fluid balance, regulation of proliferation or differentiation of breast cancer in the culture system, and the like [Gen. Pharmacol., 28, 13–22, 1997].
These proteinous and peptidergic factors, which are important for the living body, were discovered in the past, using as an indicator their inherent physiological activities. Also, by cloning techniques wherein homology to a known physiologically active protein will become a clue, similar genes having high homology have been found on their track. In order to maintain healthy conditions of the higher organisms, especially mammals, it is highly likely that humoral functional molecules other than known gene groups, which are yet unidentified by known means and their existence is yet unknown, would play some important physiological roles.
Recently, various investigations have been made with an attempt to take advantage of data processing technology using a computer and harness new gene products discovered from DNA sequencing information for biology, medicine, veterinary medicine, etc., that is, bioinformatics (Trends in Biotechnology, 14, 294–298, 1996]. Since a large scale sequencing of cDNA library becomes possible in these years, a huge number of new genes or candidates thereof come to be found by EST (expressed sequence tag) information thus accumulated. However, sequencing situation that published various databases related to the now existing cDNAs cannot cover all of the expression genes in each organism. Thus, it is not necessarily easy to further search for completely new useful gene products in these databases. On the other hand, sequencing of all DNAs possessed by one organism, namely, genome sequencing has already been completed in some species of bacteria, fungi (yeast, etc.), insects and plants; human genome sequencing is also expected to be completed in a few years. Indeed, a number of genes encoding secretory proteins or secretory peptides have been isolated so far, but in view of the entire genome, it can be hardly said to cover all of them. Since a novel substance belonging to the insulin/IGF/relaxin family will be applicable to medical treatment with high expectation, it has been strongly desired to develop such a substance.